Ochlerotatus japonicus (Theobald), a newly introduced mosquito species that occurs throughout Asia, has exhibited a remarkable range extension in just 5 years; from its introduction in the New York/New Jersey area in 1998, Oc. japonicus is now in 15 states and parts of Canada. Of special concern is the question of this species' vectorial capacity for local arboviruses; it is a competent vector of West Nile virus (WNV) in the lab and may serve as a bridge vector to humans in the northeastern US, where WNV is now endemic. However, factors affecting this species' ability to colonize new areas are poorly understood. Oc. japonicus uses a wide variety of breeding sites that encompass urban, suburban, and rural habitats, including those inhabited by Culex pipiens, the primary local vector of WNV (urban sites), and Oc. triseriatus, a native tree-hole breeder and potential vector of LaCrosse encephalitis (suburban & rural sites). This project will use traditional and novel entomological surveillance tools to quantify the effect that Oc. japonicus' introduction has on local mosquitoes' use of breeding sites and larval survival, factors that contribute significantly to a species' success in an area. Consequences of this competition will likely have broad impacts on the population dynamics of these mosquitoes as well as on the ecology of arboviruses they transmit. Our hypotheses are: 1) Oc. japonicus is a container-breeding mosquito common in wooded habitats, therefore tree holes are important breeding sites in suburban and rural areas, 2) As an opportunistic invader that has spread rapidly, Oc. japonicus will compete with native species in both adult (for breeding sites) and larval (for food) stages, and 3) Oc. japonicus is less abundant in urban areas despite numerous potential breeding sites, therefore Cx. pipiens has a competitive advantage over Oc. japonicus in urban areas. Specifically, we will: 1) Determine and compare oviposition and larval habitat preferences for Oc. japonicus and native species in urban, suburban, and rural landscapes in southern NY state, 2) Compare egg abundance and larval population densities of Oc. japonicus, Cx. pipiens, and Oc. triseriatus to determine breeding success and larval survival in each habitat type, and 3) Conduct interspecific competition experiments among these species in the lab to measure effects of density and food resources on larval survival. By comparing per capita growth rate estimates for each species, which include measurements of larval survivorship, adult size, and time from female emergence to oviposition, we will be able to predict competition outcomes in a variety of habitats. This study will be the first to examine empirically important life history parameters of this newly introduced mosquito species with respect to potential competitors. Such information is critical to understanding how a new vector becomes established in a landscape where its exposure to other circulating pathogens may facilitate its transition to a primary zoonotie vector.